1. Field of the Invention
The present invention relates to the fabrication of a cathode including microtips. It more particularly relates to the fabrication of a cathode including microtips for a flat display screen.
2. Discussion of the Related Art
FIG. 1 represents the structure of a flat display screen with microtips of the type used according to the invention.
Such microtip screens are mainly constituted by a cathode 1 including microtips 2 and by a gate 3 provided with holes 4 corresponding to the positions of the microtips 2. Cathode 1 is disposed so as to face a cathodoluminescent anode 5, formed on a glass substrate 6 that constitutes the screen surface.
The operation and the detailed structure of such microtip screens are described in U.S. Pat. No. 4,940,916 assigned to Commissariat a l'Energie Atomique.
The athode conductors are disposed in columns onto a glass substrate 10. The microtips 2 are fabricated on a resistive layer 11 that is deposited onto the cathode conductors, and are conventionally disposed inside meshes defined in the cathode conductors. FIG. 1 partially represents the inside of a mesh, without the cathode conductors. Cathode 1 is associated with gate 3 which is arranged in rows. The intersection of a row of gate 3 with a column of cathode 1 defines a pixel.
This device uses the electric field generated between cathode 1 and gate 3 so that electrons are transferred from microtips 2 toward phosphor elements 7 of anode 5. In the case of a color screen, such as represented in FIG. 1, the anode 5 is provided with alternate strips of phosphor elements 7, each corresponding to a color (red, green, blue). The strips are separated one from the other by an insulating material 8. The phosphor elements 7 are deposited onto electrodes 9, which are constituted by corresponding strips of a transparent conductive layer such as indium and tin oxide (ITO). The group of red, green and blue strips are alternatively biased with respect to cathode 1 so that the electrons extracted from the microtips 2 of one pixel of the cathode/gate are alternatively directed toward the facing phosphor elements 7 of each color.
FIGS. 2A-2D illustrate an exemplary structure of this type, FIGS. 2B and 2D being enlarged portions of FIGS. 2A and 2C, respectively. A plurality of microtips 2, for example 16, are disposed in each mesh 12 defined by the cathode conductors 13 (FIG. 2B). Here, the intersection of a row 14 of gate 3 with a column 15 of cathode 1 corresponds to 64 meshes 12 of a cathode pixel (FIG. 2A).
Cathode 1 is generally constituted by layers successively deposited onto the glass substrate 10. FIGS. 2C and 2D are partial cross-sectional views along line A-A' of FIG. 2B. A conductive layer 13, for example made of niobium, is deposited onto the substrate 10. The layer 13 is etched according to a column pattern 15, each column defining meshes 12 that are surrounded with cathode conductors 13. A resistive layer 11 is then deposited on the cathode conductors 13. The resistive layer 11, constituted for example of phosphor doped amorphous silicon, is intended to protect each microtip 2 against excessive current upon triggering of a microtip 2. The interposition of such a resistive layer 11 aims at homogenizing the electric emission of the microtips 2 of a pixel of cathode 1, thereby increasing the lifetime of cathode 1. An insulating layer 16, made for example, of silicon oxide (SiO.sub.2), is deposited onto the resistive layer 11 to insulate the cathode conductors 13 from gate 3 (FIG. 2D). The gate 3 is made of a conductive layer, for example niobium. Holes 4 and wells 17 are etched in layers 3 and 16 respectively to accommodate the microtips 2 that are made, for example, of molybdenum.
The deposition of microtips 2 in wells 17 is conventionally obtained through sputtering of molybdenum onto a lift-off layer deposited on gate 3.
A drawback of conventional techniques is that, although the resistive layer protects the microtips against overcurrents, it cannot fully homogenize the electron emission. In fact, all the microtips of a given mesh are not equidistant from the cathode conductors, which causes a lack of uniformity of the electron mission.
A further drawback lies in the difficulty of forming a meshed structure in the cathode columns. This imposes the fabrication of a complex pattern onto the whole cathode surface.
In addition, the small diameter of the microtips (ranging from 1 to 2 .mu.m) and the need for reproducing them with a high density per pixel (several thousand per pixel) causes a limitation of the possible surface area of the flat display screens. The differences that may occur in the regularity of the diameter of the holes and wells intended to accommodate the microtips are also detrimental for the homogeneity of the electron emission, by causing differences in the diameter and height of the microtips.